Since their introduction in early nineties [1] quantum well solar cells have shown great promise toward the realization of more efficient single junction and multi-junction devices [2-4].
In particular recent detailed balance calculations predict a 1-sun efficiency limit for a quantum photovoltaic cells of 44.5% [5], significantly higher than the Shockley-Queisser limit of ˜31% for homo-junction cells [6]. Such analyses assume a complete collection of all photo-generated carriers.
Whereas for relatively shallow wells (<200 meV) experience shows that the thermoionic carrier escape rates approach unity [7], for deeper wells, needed for such realizations, thermionic escape times exceed typical recombination times and photo-generated carriers may largely recombine prior to escaping the well potential. Furthermore, under the scenario of an inefficient escape process, the incorporation of multiple quantum wells, necessary for sustaining a strong photo-absorption, would also affect detrimentally the collection of carriers that emanate from the base and the emitter of the device. The comparative advantage of extending the absorption spectrum of a solar cell towards the infra-red, through the inclusion of quantum confined structures, may then be completely suppressed due to a highly inefficient collection process.
Quantum mechanical tunneling and the thermally assisted quantum mechanical tunneling represent other possible escape mechanisms for carriers photo-created in the wells. For a typical device where a set of periodic quantum wells are inserted within the intrinsic (i) region of a p-i-n diode, a direct quantum mechanical tunneling of carriers out of the wells to the continuum (through field-induced barrier triangularization) would also require shallow confinements thereby once again restricting the use of deeper wells. Nevertheless, in theory, under a large bias this effect can be leveraged to make carriers resonantly cross several successive well potentials as long as the corresponding confinement energies are more or less aligned (within +/−kBT) at the operating conditions. Unfortunately, under operating conditions, the magnitude of the electric field across the i-region is weak and hence insufficient to favor an efficient direct tunneling.
An alternate to the direct carrier tunneling is the resonant thermo-tunneling where in a succession of well/barriers the carriers are thermally excited to higher confined levels and then resonantly coupled with a shallower confined state of adjacent wells until complete extraction to the continuum. A major difficulty in realizing such a quantum well staircase design for a solar cell device resides in the engineering of a structure where alignments of confined levels between adjacent levels occur simultaneously for both electrons and holes, an almost impossible task for most quantum well material systems that exhibit strong band discontinuities both for holes and electrons.